Controller for a pulsed servovalve

ABSTRACT

An exhaust and pressure solenoid is operated by a circuit which generates a repetitive sawtooth signal waveform upon which is superimposed an input representing a desired control signal for regulating the flow of an operating fluid to a device such as a tension controller or brake. The orifices of the exhaust and pressure solenoids are duty cycle modulated at the repetitive frequency of the sawtooth signal and their respective orifice sizes are varied in accordance with the input signal.

O United States Patent 1151 3,659,631 Rakoske 1 May 2, 1972 CONTROLLER FOR A PULSED Primary Examiner-Arnold Rosenthal SERVOVALVE Attorney-Watson, Cole, Grindie & Watson [72] Inventor: John Peter Rakoske, Dover, N.l-i.

[73] Assignee: igloYore Business Forms, Inc., Niagara Falls, 57 ABSTRACT I An exhaust and pressure solenoid is operated by a circuit [22] med: 1970 which generates a repetitive sawtooth signal waveform upon [21] Appi. No.: 61,386 which is superimposed an input representing a desired control signal for regulating the flow of an operating fluid to a device [52] U 8 Cl 137/596 17 251/129 251/141 such as a tension controller or brake. The orifices of the ex- 51 inner .1III:211 .rlsk'al/os, Fisk 11/10 halls n Pressure slenids duty cycle "wdulated at the 581 Field of Search ..2s1/141, 129, so; 137/487.5 repetitive frequency of the Sawtooth Signal and their respective orifice sizes are varied in accordance with the input signal. [56] References Cited UNITED STATES PATENTS 7 Claims, 4 Drawing Figures 2,741,088 4/1956 Andrews et al. ..25i/14l X 38 +i4 ,+I'7.V

Patented May 2, 1972 5X D v zaak, Q M flaw v 1 ATTO/FA/EYS This invention relates to control systems and, in particular, to a circuitry for generating signals to control the effective orifice size of a solenoid servovalve wherein the valve member is duty cycle modulated or oscillated.

The control circuitry of this invention actuates servovalves for controlling the ingress and egress of a hydraulic ora pneumatic fluid by digital techniques without the need for converting digital signals into analog control signals. In conventional servovalve control systems, a mechanical member is moved from one position to another position to effect a change of the orifice size.

A primary advantage of such a digitally controlled servovalve is the elimination of the need for digital-to-analog conversion a paratus so that such a valve may be directly controlled by digital signals emitted from a control device. The control system enables simple on/off valve elements to be used, thereby enhancing reliability because the failure of one element results only in degraded over-all servovalve performance.

A feature of the control system is that duty cycle modulation is used to obtain infinite resolution of the effective orifice size from zero to its maximum opening. That is, an orifice of I one thirty-secondth sq. in. which is closed 75 percent of the time and open 25 percent of the time has an effective average orifice size of one one hundred-twenty-eighth sq. in. Therefore, since the on/off element may be fully closed or fully opened, or o erated at any ratio of closed time to open time, it can be operated at any effective average orifice size from zero to the maximum opening, for example, one thirty-secondth sq. in. The infinitely variable regulation of the duty cycle advantageously provides a very high resolution of the effective orifice size, and therefore the control in accordance withthis.

invention may be used in a proportional closed loop or servo system.

Further, if the frequency modulation in the control system is higher than the frequency response of other elements-in the system, then the duty cycle modulated servovalve may provide system performance equal to a conventional servovalve with respect to frequency response, deadband, friction, drift, and resolution.

In accordance with the invention, a generator provides a sawtooth waveform output at a fixed frequency. In an alternative embodiment the frequency of the sawtooth waveform may be modulated by a sweep frequency. The sawtooth wavefonn is combined with a control signal which maybe analog, digital, a.c., d.c., or a hybrid signal. The control signal varies the reference level of the sawtooth waveform and the combined signal is split and fed to each of two similarly constructed channels. The signal in each channel is compared with a reference threshold signal which is of different amplitude in each channel. The reference level difference is preferably selected so that one reference level forms an upper limit and the other reference level forms a lower limit for the combined signal. The signal in each channel is compared with its respective reference threshold level and if the signal level exceeds the reference threshold level, the resultant output is used to excite the control coil of a servovalve in each channel. This operation produces a pulsing or dithering of the valve in each channel and appropriate control of each valve by duty cycle modulation.

It is an object of this invention to provide an improved control circuit for regulating the effective orifice opening of a servovalve.

It is yet a further object of this invention to provide an improved circuit for controlling a servovalve to improve its drift and resolution and to decrease its deadband and friction.

It is still another object to provide an improved circuit of the type specified for controlling a pneumatically or hydraulically operated system by modulating the duty cycle of a servovalve member.

And yet a still further object is to reduce or eliminate beating between the modulated control signals and other frequencies existent-in the type controlsystems to which the invention relates.

. cuitry;

FIGS. -2z'z-2c'illustrate the relationship between the combined sawtooth waveform'and input control signal and the reference level in each of two channels of the circuit illustrated in FIG. 1;

FIG. 3 shows a sweep'generator circuit which may be used to drive the circuit of FIG. 1 in a modified embodiment of the invention; and

FIG. 4 is a cross-sectional view of a servovalve which is controlled by the circuit illustrated in FIG. 1.

With' reference to FIG. 1, the conduction of transistors 10, 12 produces a rising potential at 14 through resistor 16 to ground. The collector current of transistor 12 charges capacitor 18 to produce a ramp voltage input to the base 20 of driver transistor 22. When capacitor 18 charges to the firing potential of unijunction transistor 24, it produces a positive spike on resistor 26, thereby discharging capacitor 18. Transistors 10 and 12 remain conductive. Transistor 10 is a temperature compensating transistor for the current source provided by transistor 12. Capacitor 18 discharges through resistor 26 and the aforementioned cycle is repeated as the potential at 14 becomes sufficiently negative to render transistors l0, 12 conductive. The ramp or staircase signal across capacitor 18 is amplified by transistor 22 and provided to voltage divider 28 from the emitter 30 of transistor 22 via emitter resistor 32 and divider resistor 34. The components of the sawtooth generator circuit are selected to provide thedesired sawtooth waveform output signal to divider28in' accordance with characteristics of the system. For example, the frequency of the sawtooth wavefon-n may be in the range consistent with the characteristics of the servovalve.

A signal input, for example, from a controller, is applied at terminals 36, 38. Terminal '38 is grounded and the input signal at 36 is superimposed with the-aforedescribed sawtooth signal at junction 40of voltage divider 28 through resistor 42. The combined singal is split into two channels and fed respectively to high gain operational amplifiers 44, 46 through resistors 48, 50. The networkof resistors 52, 54, 56, 58, 60, 62 establishes different thresholdoperating potentials 49,51 for each of high gain amplifiers 44, '46, respectively. Thus, high gain amplifiers 44, 46 function as comparator circuits in an on/off mode. The positive input to comparator 44 is connected to threshold potential 49 as is the negative input to comparator 46 connected to threshold potentialSl. The combined input signal is then respectively applied to thenegative and positive inputs of these same comparators. Thus signals more positive than threshold potential 49 are compared by comparator 44 and signals more negative than threshold potential 51 are compared by comparator 46.

The output from comparator 44 is supplied to transistor 64 via resistor 66 and diode 68. Transistors 64 and 70 are connected in a well-known Darlington circuit to provide current amplification of the output from comparator 44 to drive coil 72 of solenoid 74. Resistor 76 and diode 78 are connected in parallel across coil 72 to dampen fly-back voltages across coil 72. Similarly, the output from comparator 46 is fed to transistor 80 through resistor 82 and diode 84. Transistors 80 and 86 also are connected in a Darlington circuit to provide current amplification of the output of comparator 46 to drive coil 88 of solenoid 90. Resistor 7 same function as described above for resistor 76 and diode 78. Solenoid 74 represents an exhaust solenoid and solenoid 90 may represent a pressure solenoid whereby pneumatic or hydraulic fluid may be exhausted from, or applied to, a desired system such as a brake for a controller in accordance with the signal input from the controller, at 36.

92 and diode 94 perform the FIG. 2a represents the sawtooth waveform in the case where the input at 36 is zero and, as illustrated, the waveform lies between threshold potentials 49, 51. The reference threshold potentials may be raised or lowered to obtain different operating conditions of the controlled servovalve mechanisms. For example, lowering each of the threshold potentials increases the duty cycle operation of the servovalves, whereas increasing the threshold potentials causesthe duty cycle to decrease.

It is apparent that the sensitivity and deadband are also afcycle modulation of solenoid 90. In an actual system the input signal at point 36 of FIG. 1 will vary in accordance with a desired control condition of the system operated by actuation of solenoids 74, 90. Thesignal input at 36 may be analog, or an a.c. or d.c. signal, or a hybrid signal representing a combination of signal types.

FIG. 3 illustrates a sweep generator circuit which may be used to modify the sawtooth waveform at 40 in the following manner. There are applications in which it is desirable to reduce or eliminate the effects of beating between the frequency of the sawtooth waveform with a resonant or other characteristic condition of the circuitwhich is controlled by the circuit of FIG. 1. Beating may be reduced or eliminated by sweeping the sawtooth output from, for example, to Hz at a l Hz rate. The sawtooth generator of FIG. 1 will produce such an output if a ramp input of 1 Hz repetition rate is applied to control the conduction of transistors 10 and 12. The 1 Hz repetitive sawtooth waveform may be produced by the circuit illustrated in FIG. 3. This circuit functions essentially in the same manner as the previously described sawtooth generator wherein transistors 100 and 102 charge capacitor 104 to provide a ramp input to the base 106 of driver transistor 108. Unijunction transistor 110fires when the potential at capacitor 104 reaches its firing potential and discharges capacitor 104, with the resultant positive spike on resistor 107. Transistors 100, 102 remain conductive; transistor 100 provides temperature compensation, and transistor 102 is a constant current source. The aforementioned cycle is repeated and the components may obviously be selected to provide the necessary 1 Hz repetition rate. The output from the emitter of transistor 108 is applied as the sweep generatorinput to the sweep generator circuit of FIG. 1 at point 114..

FIG. 4 shows, a typical solenoid or servovalve which is represented by solenoid 74, 90 in FIG. 1. A pneumatic or hydraulic input is provided at port 120 of solenoid 122 and an exhaust is obtained at port 124. Magnetically inductive slug 126 includes valve seat 128 which is spring biased by spring 130 to seat against opening 132. Slug 126 is withdrawn from seat 128 by the application of a current to coil 134. Application of a pulsating current to coil 134 will cause slug 126 and valve seat 128 to oscillate or be duty cycle modulated, thereby effectively controlling the orifice opening at 132.

Typical component values and types for the circuit of FIG. 1 are listed in Table I.

Resistor 42 18K Ohms Resistor 48 100K Ohms Resistor 50 100K Ohms Resistor 52 100K Ohms Resistor $4 387 Ohms 1% Resistor 56 13K Ohms 1% Resistor 58 10K Ohms 1% Resistor 60 332K Ohms 1% Resistor 62 100K Ohms Resistor 64 4.7K Ohms Resistor 67 2.2K Ohms Resistor 69 560 Ohms Resistor 76 100 Ohms Resistor 82 4.7K Ohms Resistor 83 560 Ohms Resistor 85 2.2K Ohms Resistor 92 100 Ohms Transistor l0, l2 2N49l7 Transistor 22 2N5306 Transistor 24 2N 2646 Transistor 64, 80 2N656 Transistor 70, 86 2N3055 Amplifier 44, 46 2Pl5A output/input 50K Capacitor 18 .47 uf, 50V

Diode 68, 84 1N4148 Diode 78, 94 1N4820 Solenoid 74, 90 Allied Control Co., Inc.

In a practical application, illustrative of the use of the aforedescribed control circuit, solenoids 74, 90 respectively are connected to the exhaust and pressure ports of a brake mechanism which is controlling the tension of a paper feed roller for a printing press. The brake mechanism is controlled by the aforedescribed operationof solenoids 74, 90 to regulate the egress and ingress of air to the brake mechanism in accordance with input signals at 36 (FIG. 1) from a tension controller associated with the paper feed roll. Such apparatus is well known to those skilled in the art and requires no further description to enable the control circuit of this invention to be used. Other applications of the control circuit, as well as modifications of the circuit described herein, will be apparent to those having familiarity with control system techniques.

What is claimed is:

1. A circuit for controlling the application of fluid to a device operated thereby in response to input signals, compris- 8;

means for generating a repetitive referencesignal having a sawtooth waveform,

means responsive to said reference signal and said input signals to form therefrom a combined signal,

means for comparing said combined signal with a threshold reference level to produce an intermediate signal when said combined signal exceeds said threshold level, said means for comparing includes two channels each responsive to said combined signal and'each having different threshold reference levels so that each channel operates on a different polarity of said combined signal, one of said channels controls the egress of fluid and the other channel controlling the ingress of fluid, and

means controlled by said intermediate signal to regulate the flow the fluid by varying the duty cycle operation of said controlled means.

2. A circuit as in claim 1 wherein said means for generating includes means for modifying the repetition rate of said reference signal at a predetermined sweep frequency.

3. A circuit as in claim 1 wherein the difference in the amplitude of said threshold reference levels is substantially equal to the amplitude of said repetitive reference signal.

4. A circuit as in claim 3 wherein said controlledmeans in.- cludes a solenoid operated valve in each channel and having a control coil for actuating a valve member to open or close a fluid inlet or outlet in response to the intermediate signal in each channel.

5. A circuit as in claim 4 wherein said means for generating includes a cyclic current generator, means for integrating the current from said current generator, and means responsive to said integrated current for determining the repetitive operareference signal from an open to a closed position in ac- 5 cordance with said input signal thereby varying the effective orifice size of said valve members.

7. A circuit as in claim 6 wherein the effective orifice established by said duty cycle modulated valve members is determined by the time interval during which said reference signal exceeds said threshold reference level in each of said channels. 

1. A circuit for controlling the application of fluid to a device operated thereby in response to input signals, comprising; means for generating a repetitive reference signal having a sawtooth waveform, means responsive to said reference signal and said input signals to form therefrom a combined signal, means for comparing said combined signal with a threshold reference leVel to produce an intermediate signal when said combined signal exceeds said threshold level, said means for comparing includes two channels each responsive to said combined signal and each having different threshold reference levels so that each channel operates on a different polarity of said combined signal, one of said channels controls the egress of fluid and the other channel controlling the ingress of fluid, and means controlled by said intermediate signal to regulate the flow the fluid by varying the duty cycle operation of said controlled means.
 2. A circuit as in claim 1 wherein said means for generating includes means for modifying the repetition rate of said reference signal at a predetermined sweep frequency.
 3. A circuit as in claim 1 wherein the difference in the amplitude of said threshold reference levels is substantially equal to the amplitude of said repetitive reference signal.
 4. A circuit as in claim 3 wherein said controlled means includes a solenoid operated valve in each channel and having a control coil for actuating a valve member to open or close a fluid inlet or outlet in response to the intermediate signal in each channel.
 5. A circuit as in claim 4 wherein said means for generating includes a cyclic current generator, means for integrating the current from said current generator, and means responsive to said integrated current for determining the repetitive operation of said cyclic current generator to establish said repetitive reference signal.
 6. A circuit as in claim 5 wherein each of said valve members is duty cycle modulated at the repetitive rate of said reference signal from an open to a closed position in accordance with said input signal thereby varying the effective orifice size of said valve members.
 7. A circuit as in claim 6 wherein the effective orifice established by said duty cycle modulated valve members is determined by the time interval during which said reference signal exceeds said threshold reference level in each of said channels. 